Probing Swelling and Molecular Conformation on Polymeric Coatings for Biocompatibility

INTRODUCTION The approach-retract (A-R) cycle (“force curve”, “force spectroscopy”, “force calibration mode”, etc.) has become indispensable to scanning force microscopy. The need to control load and measure tip-sample adhesion (e.g. to monitor the state of the tip) requires routine use of this measurement mode. Many researchers have gone beyond these simple applications to probe the distancedependent “fingerprint” of interfacial forces in liquid media, even at the level of single molecular interactions. The further capability to spatially map (X-Y) the Z dependence of the interaction enables one to image and survey interactive fingerprints across a surface. This is important even on a homogeneous surface, in order to provide enough measurements to determine error bars and average over topographic variations. On heterogeneous surfaces, A-R mapping not only reveals spatial variations but also provides information that can be essential to properly interpret topographic images, i.e. corrective (e.g. compliance accounting) and diagnostic information (e.g. determination of net attractive or repulsive dynamic interaction). The present study is a first attempt to exploit the above attributes of A-R cycles to investigate swelling and molecular conformation on ultrathin films of polyacrylamide, a coating used to modify the surface of medical devices prior to insertion into the body. To measure swollen film thickness we clear the substrate via abrasive scanning, zoom out and image topography at nonperturbative loads, and correct the apparent depth of the cleared trench by accounting for compliance as quantified in A-R cycles. Beyond this straightforward exercise, we examine the details of A-R cycles and survey conformational states. Measurements during retraction identify an ensemble of contour lengths atop unperturbed film. Film displaced by previous abrasive scanning exhibits conformations distinctly different from unperturbed polymer, and further varies depending on location relative to the perturbative raster-scan pattern. We suggest broader applications of our methodology towards a variety of problems in both fundamental and applied polymer research using scanning probe microscopy.